1. Field of the Invention
This invention relates to improvements in a structure for holding lenses in an optical device.
2. Description of the Related Art
FIG. 5 is a sectional view illustrating the internal construction of an optical displacement detector. The detector includes a casing 20 within which are provided a light-emitting diode 21 serving as a light source, a pair of ball lenses 22, 23, a half-mirror 24, a photodiode 25 for monitoring purposes, and a photodiode 26 for producing a signal. Also provided within the casing 20 are actuators 30 capable of being freely advanced and retracted in a direction orthogonal to the optic axis of this optical system. The actuator 30 is biased outwardly at all times by a spring and is held at its outermost position by a stopper. A bellows is provided on the outwardly projecting exterior of the actuator 30. The inner end of the actuator 30 is formed into a knife edge.
When the actuator 30 is inwardly displaced as a result of being pushed by an object, the knife edge cuts off part or all of the light directed from the ball lens 23 to the photodiode 26. As a result, the level of the output signal from the photodiode 26 changes. Thus, there is obtained a signal representing the amount of displacement of the actuator 30, namely a signal representing the amount of movement of the object acting upon the actuator, the external shape of the object, etc.
The light-emitting diode 21, ball lenses 22, 23, half-mirror 24 and monitoring photodiode 25 constituting the internal elements of the detector are actually fitted into an internal casing 40 and are integrated to form an optical device.
A problem encountered in the prior art is that the optic axes of the elements in the abovementioned optical device cannot be aligned accurately.
Specifically, FIG. 6 illustrates, in simplified form, the arrangement of the two ball lenses 22, 23 in the internal casing 40 as seen from the side thereof. In order to clarify the problem with the prior art, the shape of the internal casing 40 is simplified in comparison with that shown in FIG. 5. The internal casing 40 comprises a base portion 41 to which each element is attached, and a cover portion 42 provided on the base portion 41. The base portion 41 is provided with a groove 43, and the two ball lenses 22, 23 are fitted in the groove 43 at predetermined positions and are fixed to the groove by a bonding agent.
Accordingly, in a case where the groove 43 is formed to the same depth in a regular shape, an optic axis Ra connecting the centers of the two ball lenses 22, 23 is situated in a horizontal attitude. However, if the groove 43 is formed to have an incline at the time of manufacture, as indicated by the two-dot chain lines, one ball lens 22 will be lower than the other and the optic axis Rb will assume an inclined state. When the optic axis of the ball lenses 22, 23 is skewed in this manner, the light from the light-emitting diode 21 does not reach the photodiode 26 in a normal state and the monitoring operation performed by the photodiode 25 also does not take place normally.